I think your view on flash is a little too shallow, which also applies to the many answers provided here. I have been working with the Flash Player as a platform for about 5 years now and I would like to take the time to explain a few things.
TLTR: goto conclusion.
The problem with Flash
One of the biggest problems of that platform is how it is monetized:
By providing development tools that lower the entry barrier to a bare minimum. As a result, you have a shitload of crappy apps out there.
What is claimed about flash is true for flash content, but is not specific for the platform. The same company, that created flash, created Dreamweaver. Last time I checked, it produced results about as crappy as average flash content.
The "Truth" about Flash
- Flash Player's market penetration can measure up with that of browser you'd be willing to actually be compatible with when creating a HTML solution. Estimations for IE6 still range up to over 10%.
- Flash Player is not present on mobile devices, at least not in a way one could seriously work with. OTOH, if you honestly care about mobile experience, then you should create a mobile version of the site you create.
- Flash Player is used to create crappy multimedia sites and banners. However, Flash Player is capable to run rich internet application such as Sliderocket, Aviary Tools (notably Phoenix), games such as Tanki Online and Koyotl (I myself am working on this game and we'll hopefully launch a public beta next week, until then the screen shots at the bottom should give you an impression).
There is such a thing as actual flash development and if you're actually interested, I suggest you grab a copy of FlashDevelop which is all about actual software development on the flash platform.
Your claim that flash requires more development time is solely based on your personal skillset. It's as if a .NET developer claimed Java development requires more time.
Flash vs. HTML
Flash is - from my personal experience - a better choice than HTML, if you want to build a complex, persistent, interactive app, a fat client that deploys from the web (*).
Now to build web sites with a relatively low level of interactivity, as for example a forum, there is not much reason to use flash, except some edge cases that are not really important here. Choosing HTML makes it easier to do SEO and to ensure accessibility. But it is still a matter of taste, although I cannot imagine non-technical people having any good reason to prefer flash.
Depending on the situation and personal philosophy, one or the other is "better", but statements from "flash sucks" to "it is ok as a complement" are oversimplyfied.
What I fear is, by wanting pretty, your customers mean the kind of pretty that makes you wanna vomit. In that case flash also is the better tool to implement this nightmare. I turn down such jobs and I also don't forward them. This is a crime. And I am afraid once the HTML5 backend of Adobe CS Flash is stable enough, you will not be able to hide from this just by uninstalling flash.
If it is that what they want, I beg you to try talking them out of that. It's not hip, it's not cool, it's distasteful. You know that. Everybody here does. They should, too.
Demonizing flash isn't really honest and I don't think you need it.
The best work you can do at minimum cost is to use the technologies you master and your customers want to get just that. Unlike a number of popular half-truths this is a solid fact.
(*) : I think standards are a lovely thing. I myself am using haXe since 2 years now, one motivation being the ability to switch, when I consider HTML a working alternative for my requirements. So far I must say, the flash player is still my prefered target for a number of reasons of which I presented a subset. So while I am looking forward to the day when HTML makes the flash player obsolete for my work, I think it's still a few years from now.